Regression: clicking tab in TUI/GTK no longer highlights matching file in explorer tree (only expands its folder)

[JDonaghy]

Symptom

Clicking a tab in either TUI or GTK previously did two things:

1. Expanded the parent folder in the explorer tree (if it wasn't already expanded).
2. Highlighted the corresponding file row as the tree's selection.

Now only #1 happens — the folder expands but the file row is not highlighted as selected.

Probably an engine-level regression in the click handler (or a missed `selected_path` write on the path that transitions through tab-click → reveal-in-tree). Both backends affected so the bug is engine-side, not in the rasteriser.

Repro

1. Open multiple files from different folders.
2. Click on a tab whose file lives in a folder that's currently collapsed.
3. The folder expands; the file row is not the tree's highlighted selection.

Expected: file row also gets the highlighted-selection styling.

Likely files

• `src/core/engine/buffers.rs` — search for `reveal_in_tree` / similar tab-activation hooks.
• `src/core/engine/windows.rs` — `set_active_tab` / `activate_tab`.
• `src/render.rs` — `explorer_to_tree_view` to verify it derives `selected_path` from the engine state correctly.

The rasteriser side (`quadraui::tui::draw_tree` + `quadraui::gtk::draw_tree`) is fine — it highlights whatever row's path matches `tree.selected_path`. So the bug is in the engine's failure to set `selected_path` on tab activation.

Surfaced by

#223 Dialog pilot smoke test (Session 332). Pre-existing — not introduced by the rasteriser pilots.